Method for treating surfaces of zirconium alloy tubes

ABSTRACT

A method for producing a highly corrosion resistant surface on a zirconium-base alloy tube by abrading away with a fine abrasive a contaminated surface layer of the tube to provide an initial striated surface with successive ridges and grooves, and then abrading the initial surface with a functionally finer abrasive to a smoother surface with successive ridges and grooves.

United States Patent 1191 Shaffer et al.

METHOD FOR TREATING SURFACES OF ZIRCONIUM ALLOY TUBES Inventors:Frederick Shaffer, Columbia, S.C.; David B. Scott, Manchester, Conn.

Assignee: Westinghouse Electric Corporation,

Pittsburgh, Pa.

Filed:. July 12,1971 Appl.No.: 161,771

Related US. Application Data Continuation-impart of Ser. No. 678,996,Oct. 30, 1970, abandoned.

us. c1 s1/326,51/139, 5l/328 1111. C1 B24b 1/00 Field of Search 51/289R, 328,. 139, 326,

References Cited UNITED STATES PATENTS Oas 51/139 hi1 l I ll:

Primary Examiner-Donald G. Kelly Attorney, Agent, or Firm-F. ShapoeABSTRACT A method for producing a highly corrosion resistant surface ona zirconium-base alloy tube by abrading away with a fine abrasive acontaminated surface layer of the tube to provide an-initial striatedsurface with successive ridges and grooves, and then abrading theinitial surface with a functionally finer abrasive to a smoother surfacewith successive ridges and grooves.

1 1 1 I'll i i'i 1 6 Claims, 3 Drawing Figures 1 111 1 1' /24 111 1 1 1101 1 1 111 1 MW 1 11 l i i 111 11* 11 111111 1.111111.111,1 11 11 11 11 111111 METHOD FOR TREATING SURFACES OF ZIRCONIUM ALLOY TUBES CROSSREFERENCE TO RELATED APPLICATIONS This application is acontinuation-in-part of our application Ser. No. 678,996, filed Oct. 30,1967, now abandoned and is related to application Ser. No. 493,845, nowabandoned.

BACKGROUND OF THE INVENTION 1. Field of the Invention This inventionrelates to a method for preparing zirconium alloy members, particularlymetal tubing with a highly corrosive resistant surface, and moreparticularly, it pertains to a method for preparing zircaloy tubing foruse in a nuclear reactor in which the tubing is in contact with hightemperature water or steam.

2. Description of the Prior Art Zircaloy is a generic designation forcertain zirconium-base alloys that are usefulin the nuclear industry dueto their low neutron capture cross section, good mechanical strength,heat resistance, and corrosion resistance to high pressure and hightemperature water and steam. Because of their low neutron capture crosssection, zirconium-base alloys are useful as structural materials andfuel element cladding. Zircaloy is available as zircaloy-2 andzircaloy-4. The nominal composition in weight percent of zircaloy-2 is1.5% tin, 0.12% iron, 0.10% chromium, 0.05% nickel, and the balancezirconium. The nominal composition of zircaloy-4 is 1.5% tin, 0.21%iron, 0.10% chromium, less than 0.007% nickel and the balance zirconium.Zircaloy compositions are set forth in US. Pat. No. 2,772,964.

In order to provide the alloys with a maximum of corrosion resistance ithas been necessary to subject the alloy to a prescribed picklingprocedure for surface conditioning. That procedure has heretoforeinvolved pickling in an aqueous solution of hydrofluoric and nitricacids followed by a prolonged cleaning operation. The pickling operationhas been necessary to remove defects in the surface of the zircaloytubing which are the source of premature corrosion during use. However,the pickling operation has created an additional problem because of thedifficulty in rinsing off fluoride salts after pickling. The presence ofthese salts, particularly zirconium oxyhexafluoride (Zr(OI-I)F' has beenresponsive for the accelerated formation of white corrosion products anda generally unacceptable zircaloy tubing. Detection of the whitecorrosion product involves a time-consuming procedure of autoclaving inwater or steam and inspection as a prerequisite to approval to approvalof each pickeled tube. The cost of the pickling and autoclaving andcorrosion inspection operations therefore have been material items infuel elements and the fabrication of cores for nuclear reactors.

A method for avoiding the foregoing difficulties by eliminating thecorrosion testing operation is set forth incopending application Ser.No. 493,845 (now abandoned) disclosing a method for abrading andburnishing the tube surface whereby a contaminated surface layer oftubing is removed by abrading the surface, and, after cleaning theabraded surface to remove any abrasive grit particles, the abradedsurface is burnished to roll out the roughened or straited surfacecreated by the abrading step. That procedure is satisfactory so long asthe outer diameter of the tube is substantially uniform from one end'tothe other. For example, zircaloy tubing having a diameter of 0.423 inchmay have a tolerance of i 0.015 inch as received from the manufacturer.Unfortunately, the tolerances of tubing as received from a manufactureroften vary too widely to permit the fully successful use of the abrasionand burnishing procedure as set forth in said patent application Ser.No. 493,845 (now abandoned). Since burnishing tools have rigid rolls andcan accommodate only slight variation in diameter, where the outerdiameter of the tubing is larger than that for which the tool isdesigned, the burnishing operation causes work hardening of the outerdiameter layer and thereby introduces a camber or bow into the tubinginvolved. A cambered or bowed tube is undesirable because it cannot beloaded with pellets of nuclear fuel. On the other hand where the outerdiameter of the tubing is smaller, the burnishing rolls fail to smoothout the serrated surface or ridges or grooves caused by the priorabrasion operation and in some cases may even fail to contact the tubewall. As a result the unburnished or lightly burnished areas aresusceptible to a high corrosion rate.

Associated with the foregoing is a problem of eccentricity or ovality towhich a burnishing tool is nonselfadjusting to accommodate thenon-circular cross section.

In accordance with this invention, it has been found that the foregoingdifficulties may be overcome by the use of a double abrading procedurewhereby an outer surface layer of tubing is abraded away by an abrasionstep using fine abrasive, after which the abraded surface consisting ofstriations is removed by a subsequent abrasion step of a finer abrasiveparticle, leaving a finished surface having striations characterized bysuccessive grooves of less depth than those provided by the previousabrasion step.

Accordingly, it is a general object of this invention to provide amethod for the mechanical surface preparation of zircaloy for avoidingsurface corrosion.

It is another object of this invention to provide a method ofmechanically preparing the surface of zirconium base alloy tubing forcorrosion resistance without burnishing.

Finally, it is an object of this invention to provide a method for thesurface preparation of zirconium base alloy tubing which accomplishesthe foregoing objects and desiderata in a simple and effective manner.

SUMMARY OF THE INVENTION BRIEF DESCRIPTION OF THE DRAWINGS For a betterunderstanding of the invention reference is made to the drawings, inwhich:

FIG. 1 is a diagrammatic view showing the application of two separateabrading belts to the tube surface;

FIG. 2 is a highly enlarged diagrammatic sectional view through asegment of tubing showing the original, intermediate, and final surfaceconfigurations before and after the two abrasion steps; and

FIG. 3 is a graph showing the weight gains of zircaloy-4 tubing orcontrol coupons after exposure to 750F steam for up to 42 days.

DESCRIPTION OF THE PREFERRED EMBODIMENT Usually tubing composed ofzirconium-base alloy as received from a manufacturer has surfacecontaminants, principally fluorides and oxides as well as pits andscratches, resulting from previous hot and cold processing, includingpickling, and handling, which defects are conducive to corrosion if notremoved. In FIG. 1, a tube of zirconium base alloy is mounted for aso-called double abrading of the outer surface with an abrading meanscomprising a pair of endless abrasive belts 12 and 14. Both belts l2 and14 move in the same direction, as indicated by the arrows 16, so thatthe tube 10 is rotated by virtue of its contact with the belts.Moreover, the tube is mounted on support rollers 17 the axes of whichare slightly inclined to cause the tube 10 to advance to the right asviewed in FIG. 1. Thus, the tube 10 rotates and advances longitudinallywith respect to the abrasive belts as indicated by the direction arrow18. For conventional tube of an outside diameter of 0.423 inch, thespeed of the tube across the belts l2 and 14 is approximately 17 feetper minute but may be varied as deemed necessary.

As the tube 10 rotates and advances in contact with the belt 12 theouter peripheral surface 20 (FIG. 2) of the tubing is removed leaving aninitial relatively roughened surface 22. The second belt 14, having afunctionally finer grit size than that of the belt 12, acts as asmoothing opera-tion and removes the roughened surface 22, leaving asmoother clean surface 24 which is devoid of successive high ridges 26and deep grooves 28 which characterized the surface 22.

During the initial abrasion by the belt 12 a surface layer of about from0.0002 to 0.0004 inch thick is removed. Consequently approximatelyone-half mil is removed from the overall diameter of the tube. The belt12 is preferably provided with an abrasive, such as silicon carbide(SiC), comprising particles which pass through a standard screen havingfrom about 200 to 500 openings per lineal inch. The belt comprises acloth backing to which the abrasive particles are attached.

The belt 14 comprising a functionally finer abrasive removes from about0.0001 0.0002 inch of surface layer from the tube 10. In other words,from about oneeighth to one-fourth mil is removed from the overalldiameter of the tube by the second abrasive belt. To accomplish itspurpose, the belt 14 is provided with a finer abrasive of the order of100 mesh finer than the initial belt, such as silicon carbide,comprising particles having a grit size of about 500 to 600 affixed to acloth backing.

A coarser abrasive, namely from 400 to 500 mesh silicon carbideparticles, if affixed to a cork or other resilient backing disposed onthe cloth belt, has been found to function as well as the 600 meshsilicon carbide abrasive affixed directly on the cloth backing.Functionally the two are equivalent, and good results have been obtainedin service with both types of belts. There should be a functionaldifference in grit size of at least between the initial coarse and thefinal fine abrasive belt.

In some cases three or more abrasive belts can be employed, providingthat the final belt is functionally equivalent to a 600 mesh siliconcarbide abrasive affixed to a cloth backing. The 600 mesh abrasive mayinclude particles as coarse as about 550 mesh.

It has been found that silicon carbide or a hard abrasive material of aKnoop hardness of over 2,000 must be used to cut away the superficialtube surface since a softer abrasive, such as aluminum oxide (A1 0deforms or smears the surface to a high degree and is not effective inadequately removing a metal surface layer from the zirconium alloymembers.

Abrasive belts with abrasive SiC of 600 mesh or greater fineness are notcommercially available, and such abrasive belts were specially made forthe practice of this invention.

Associated with the foregoing is the desirability of correlating thebelt speed and the rate of advance of the tube to obtain the desiredfinal smooth finished surface which is devoid of the high ridges 26 andthe deep grooves 28 which may give rise to problem in reactors in thatcrud may deposit on such relatively rough surface. Accordingly with atube advance speed of 17 feet per minute a belt speed of about 230 feetper second has been found to avoid spiral striations and smeared metal.

An exhaust system is applied to the zone between the belts 12 and 14 inorder to remove air borne dust and grit particles from the tube surfacebefore it advances to the final belt 14.

As an example, 0.423 inch outside diameter zirealoy- 4 tubing isinitially abraded in air with an abrasive belt comprising 400 meshsilicon carbide affixed to a cloth back-ing to remove approximately aone-half mil thickness of the outer surface of the tubing. The beltspeed is 230 feet per second and the tube is moved relative to the beltat about 17 feet per minute. This abraded tubing is then abraded with anabrasive belt comprising 600 mesh silicon carbide adhesively affixed toa cloth backing. An excellent, smooth, clean tubing surface results. Thesecond belt abrading removes less than onefourth mil of the tubesurface.

In another test the second belt comprised a thin cork base layerattached to the cloth backing and 400 mesh silicon carbide wasadhesively affixed to the cork base layer. Zircaloy tubing produced bythe second belt is of a smoothness and cleanliness similar to thatproduced by the 600 mesh belt.

Test results for the corrosion rate of zirealoy tubing and test couponsprepared by the double abrading tech-nique as set forth above, as wellas by the abrading and burnishing technique of patent application Ser.No. 493,845 (now abandoned) are shown in the following table. Thecomparisons are made with the published standard results for corrosiontests referred to as controlled coupons. In addition the standardallowable upper limit of corrosion weight gain is also included.

TABLE Condition After 14 After 28 After 42 Days Days Days Double Abraded+3l.8 +391 +482 mg/dm Abraded & Burnished +29.7 .+37.l +5l.0 ControlCoupons +23.7 +291 +45.8 Allowable Limit 38.0 max. 43.0 max. 55.0 max Itwill be understood that the tubing can be previously ground or abradedwith coarse abrasives before employing the belt abrasion techniques ofthis invention.

The smaller the weight gains for a given test the more satisfactory thetest results. Weight gains are measured in milligrams per squaredecimeter (mg/dm). Test results which are less than those shown for thepickled tubes or c0n-trol coupons that are only slightly greater thanthe pickled specimens, are deemed very satisfactory.'The results of thetable are plotted in the graph of FIG. 3 where it is evident that overextended periods of time the double abrading procedure results inzircaloy which corrodes on a substantially linear scale and after 42days testing ultimately the corrosion resistance is .equivalent to or isbetter than the prior known procedures of pickling or abrading andburnishing, as indicated by the extrapolated (broken) lines beyond the42 day test. Moreover, the double abrading method has resulted in norejections of tubing because of surface defects arising from theabrading process while a rejection rate of about 7 percent prevailed fortubing prepared by pickling and corrosion testing. Finally, visualstandards used to check the double abrading tubing indicates it to be ahighly satisfactorily controllable process.

Accordingly, the advantages of the method for double abrading over priorknown procedures provides a method for providing a zirconium base tubesurface which is highly corrosion resistant to water present in anuclear reactor, which surface is obtainable at lower cost and with lesstime and less capital equipment inve'stment than has been involved withother heretofore known methods.

The abrasive belt techniques has been found to accommodate eccentrictubes, as well as tubes of varying diameter and to adequately abrade allthe surfaces to provide improved corrosion resistance in accordance withthe invention. It should be understood that the abrasive steps may becarried out in air and this greatly reduces the expense of the process.

Liquid coolants comprising water may be flowed over the tubing as it isbeing abraded. The liquid coolant carries away the zirconium alloy dustand reduces fire hazard, and it also keeps both the belt and tubingcool.

A three belt system may be employed to process zir- 'caloy tubingwherein the first belt comprises 220 mesh silicon carbide applied to acloth backing, the second belt comprises 320 mesh silicon carbideapplied to cloth backing and the third belt comprises 400 mesh siliconcarbide applied to a cork backing carried by a cloth belt, all threebelts being liquid cooled at the point of contact with the tubing.

What is claimed is:

' l. The method of treating the surface of a zirconium base alloy tubingto render it suitable for use in a nuclear reactor in contact with hightemperature water or steam, the step comprising first abrading thesurface layers of the tubing with an abrasive belt comprising particlesof an abrasive of from about 200 to 500 mesh size, to remove at least0.0002 inch of all the outer tubing surface to provide an initialabraded striated surface comprising a series of relatively coarsecircumferential ridges and grooves, and subsequently abrading theinitial surface with a substantially finer abrasive belt having thefunctional effectiveness of a belt comprising a cloth backing havingaffixed thereto an abrasive of about 500 to 600 mesh'size so as toremove at least 0.0001 inch of the outer tubing surface, both abrasiveshaving a Knoop hardness of at least 2,000, whereby to provide a finalsmooth clean'surface having good corrosion resistance in a nuclearreactor, the abrading steps capable of being carried out in air.

2. The method of claim 1 in which both abrasive particles comprisesilicon carbide.

3. The method of claim 1 in which the zirconium base alloy is zircaloy.

4. The method of claim 1, in which the abrading is accomplished bymoving the tubing with respect to the abrasive belts.

5. In the process of producing a smooth surfaced zirconium base alloytubing suitable for use in a nuclear reactor in contact with water orsteam, the steps comprising initially abrading the zirconium alloytubing in air with a relatively coarse abrasive belt to remove at least0.0002 inch of the outer surface of the tubing, and finally abrading thezirconium alloy tubing in air with a substantially finer abrasive belt,the abrasives of both belts having a Knoop hardness'of at least 2,000.

6. The process of claim 5 in which the second belt has an abrasivefunctionally at least mesh finer than the first abrasive belt.

2. The method of claim 1 in which both abrasive particles comprisesilicon carbide.
 3. The method of claim 1 in which the zirconium basealloy is zircaloy.
 4. The method of claim 1, in which the abrading isaccomplished by moving the tubing with respect to the abrasive belts. 5.In the process of producing a smooth surfaced zirconium base alloytubing suitable for use in a nuclear reactor in contact with water orsteam, the steps comprising initially abrading the zirconium alloytubing in air with a relatively coarse abrasive belt to remove at least0.0002 inch of the outer surface of the tubing, and finally abrading thezirconium alloy tubing in air with a substantially finer abrasive belt,the abrasives of both belts having a Knoop hardness of at least 2,000.6. The process of claim 5 in which the second belt has an abrasivefunctionally at least 100 mesh finer than the first abrasive belt.